1. Field of the Invention
The present invention relates to a lithographic plate which does not require dampening water at press operation (referred to as "water-less plate" hereinafter) and, more particularly, to a photosensitive lithographic plate which does not require dampening water (referred to as "water-less PS plate", hereinafter), which is suitable for use in making the water-less lithographic plate and which is superior in sensitivity, developability, run-length (i.e. press life) and ink-receptive property, as well as in plate-examination property.
2. Description of the Prior Art
Water-less PS plates have been known in which a photosensitive layer and a silicone rubber layer are formed in sequence on a substrate. Such water-less PS plates are disclosed, for example, in U.S. Pat. Nos. 3,511,178; 3,677,178 and 3,894,873; and British Patent No. 1,419,643, as well as in U.S. Pat. No. 4,342,820.
Broadly, there are two types of method of preparing a lithographic plate from a water-less PS plate. In one method, the portion of the photosensitive layer corresponding to an ink-receptive area (referred to as "image area" hereinafter) is dissolved by a developing solution thereby to remove the portion of the silicone rubber layer on the dissolved portion of the photosensitive layer, thereby to form the image area, as disclosed in U.S. Pat. No. 3,511,178. The other method makes use of the photo-bondability and photo-releasability of the photosensitive layer in such a way as to selectively remove the portions of the silicone rubber layer corresponding to the image area, as disclosed in U.S. Pat. Nos. 3,894,873 and 4,342,820.
The first-mentioned method is advantageous in that the developability is not affected substantially even when the strength of bonding between the silicone rubber layer and the photosensitive layer is strong enough, because the image area is formed by removing the photosensitive layer. This means that, according to this type of method, it is possible to obtain a plate having improved anti-scratch and anti-wear characteristics without substantially impairing the developability. On the other hand, this type of method requires that the photosensitive layer be made as thin as possible. Namely, since the image area is formed by a recess penetrating both the photosensitive layer and the silicone layer, the depth of the recess, i.e., the depth of the image area, will become large when the photosensitive layer has a large thickness. Such a large depth of the image area will require a large amount of ink to be supplied to the plate surface, in order to fill such an image area of the large depth.
Under these circumstances, the present inventors have developed a water-less PS plate having a primer layer, a photosensitive layer and a silicone rubber llayer which are formed in the mentioned order on the substrate. In this improved water-less PS plate, the photosensitive layer has a reduced thickness so that it can operate with smaller amount of ink than the conventional water-less PS plate. In addition, this improved water-less PS plate exhibits superior anti-scratch and anti-wear characteristics. The present applicant has already filed an application for a patent on this improved water-less PS plate, as J. P. KOKAI 60-229031. In this water-less PS plate, a primer layer, a photosensitive layer and a silicone rubber layer are formed in the mentioned order on a substrate, and the primer layer consists of a photo-hardened photo-crosslinkable resin.
In the production of this water-less PS plate, the photosensitive layer and the silicone rubber layer are successively formed on the primer layer by means of suitable organic solvents. On the other hand, in the plate making process, the image area is formed by selectively removing imaging portions of the photosensitive layer and the silicone rubber layer by developing solution which contains an organic solvent. It is essential that the primer layer not be dissolved nor exfoliated due to swell. Thus, the primer layer is required to have strong resistance against solvents.
The portions of the primer layer corresponding to the image area are exposed upon the development. The exposed portions of the primer layer are dyed during or after the development so as to facilitate the examination of the plate whether the image area being precisely reproduced after the development. Therefore, it is also necessary that the primer layer can easily be dyed. Namely, the primer layer is required to have an excellent dyeability.
Thus, the primer layer has to meet both the demands for high anti-solvent characteristic and high dyeability. It is, however, extremely difficult to satisfy both these demands simultaneously. For instance, when an ordinary threedimensionally-crosslinked binder such as a cured epoxy resin is used, the primer layer has to have a high degree of crosslinkage in order to have a high resistance to solvent. While such a high degree of crosslinkage substantially prevents swelling of the primer layer, thereby the dyeability is seriously impaired. If the degree of crosslinkage is lowered for allowing the primer layer to swell substantially, i.e., for attaining a high dyeability, the primer layer can have only a low resistance to solvent. It would be possible to make the primer layer contain a functional group which is easily dyeable. Such a measure, however, is still unsatisfactory. Namely, the primer can be dyed only at its surface region and the dyed image can have only a small density which is not suitable for the plate examination.